1. Field of the Invention
This invention relates to an apparatus for separating condensable gas, especially water vapor from a gas mixture containing condensable gas, by using an inorganic porous membrane, preferably an inorganic porous hydrophilic membrane, and to a process for such separation.
2. Description of the Prior Art
The degree of sultriness which the human body experiences is influenced not only by high temperature but also by high humidity. Accordingly, there is a large demand for dehumidifiers since people feel more comfortable when humidity is low. Also, since water is an active molecule, the removal of water is indispensible for the storage of drugs and other chemicals. From a hygienic viewpoint, dehumidification is necessary because the multiplication of microorganisms such as molds is more active at high humidity. Further, humidity control is practiced in a wide variety of fields including the electric industry, precision machine industry, and textile industry.
Presently, there are three practical methods for removing water vapor. The first method involves bringing gas into contact with a hygroscopic agent, such as a silica gel, a molecular sieve, quick lime, calcium chloride, phosphorus pentoxide, lithium chloride, or concentrated sulfuric acid, to remove moisture contained in a gas. In this method, it is necessary to dispose or regenerate the used hygroscopic agent and, therefore, continuous operation is impossible when only one dehumidifier is used.
The second method involves condensing moisture contained in a gas by compressing or cooling the gas to thereby remove the moisture. This method has an advantage in that continuous operation and mass treatment are possible, but has disadvantages in that it requires a large quantity of energy and that dehumidification to low humidity is difficult.
The third method is one which has been recently developed and it involves removing water vapor from a gas by using a membrane having selective permeability to water vapor. This method includes two processes, i.e., one in which a homogeneous membrane is used and one in which a porous membrane carries a hygroscopic agent. Both of these processes have an advantage in that continuous operation is possible.
The process in which a homogeneous membrane is used provides a high separation ratio but has a disadvantage in that the permeation rate is low. When the permeation rate is low, the quantity of vapor permeated can be increased by increasing the difference in partial pressure between both surfaces of the membrane, which serves as a driving force for permeation through the membrane. However, this has been difficult because the saturated vapor pressure of water is as low as about 20 mmHg at room temperature. In reality, when a method in which the separation of water vapor is carried out by using a thin polymer film having a very high permeability to water vapor is considered, such as in Japanese Patent Laid-Open No. 152679/1979, the water vapor permeation rate is 5.6.times.10.sup.-6 g/cm.sup.2.sec.cmHg for a film thickness of 5 .mu.m, and even when a hollow fiber with a membrane area of as large as 4.7 m.sup.2 is used, the throughput is as low as 100 l/min and the dehumidification rate is as low as 79%.
The process in which a porous membrane carries an adsorbent is one involving the use of a membrane formed by impregnating a porous support, for example, paper, cloth, or nonwoven fabric, with a hygroscopic polymer, for example, polyvinyl alcohol or polyethylene glycol, and/or a hygroscopic agent, for example, lithium chloride; see for example, Japanese Patent Laid-Open Nos. 2277/1979, 114481/1979, and 119421/1980. This process can provide a high permeation rate but has a disadvantage in that the membrane contains a hygroscopic polymer or agent which absorbs water when the membrane is used or left standing under a high humidity condition so that the formed solution exudes from the membrane to lower the performance of the membrane. In the membrane separation method, it is most suitable to increase the difference in partial pressure of water vapor between both surfaces of the membrane by reducing the pressure on the effluent side, but this is thought to be impossible because the membrane does not have sufficient pressure resistance. In fact, in the above-mentioned process, the reduction in pressure is not realized and the moisture is simply exchanged between a gas mixture and dry gas. A system which is carried out by using dry gas can not provide good efficiency because dehumidification of a gas mixture of 100% relative humidity to below 10% relative humidity requires dry gas of 0% humidity in an amount of about 10 times that of the gas to be dehumidified assuming the moisture exchange is perfect.
In some methods of separating a gas, such as hydrogen or helium, a porous membrane is used. If selective permeation to water vapor can be performed by using a porous membrane only, it is thought possible to obtain a water vapor separation apparatus which has a simple structure and sufficient stability and durability because the permeation rate is high and no adsorbent is used. In general, however, gas separation by a porous membrane utilizes Knudsen flow, which is a phenomenon by which the permeation rate of a gas is inversely proportional to the square foot of its molecular weight. Therefore, though this method is effective for the separation of hydrogen, it is ineffective for separating water vapor from air, for example, since the separation ratio becomes as low as about 1.26 due to the close molecular weights of air and water. Thus the practicality is low, and there has been no actual example of water vapor separation by a porous membrane. Actually, as the following Comparative Example 1 demonstrates, air having a 62% relative humidity is dehumidified to only about 50% relative humidity in a system in which the separation is performed by Knudsen flow, and the recovery rate is as low as 16%, which is not practical.